Texas Instruments 2000 series, Reference Manual

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51
Over Voltage Protection
For applications where there is a risk that voltage spikes and noise are on the lines
to the RA-RFM, additional protection circuitry and filters must be added. A proposal
on how this may be achieved is shown in Figure 13, and this circuit may be used
as a guideline for protection circuitry. This may not be sufficient for all applications,
however, and must be checked individually when necessary.
1. The supply input has to be protected against voltage spikes. R1 and D1 fulfil
this purpose. Zener diode D1 clamps the voltage spikes to 18 volts so that the
maximum allowed transmitter power stage supply voltage is not appreciably
exceeded. For diode D1, type ZY18 is recommended, this type having a 2 W
power dissipation. If a higher current is needed, dump type ZX18 may be used,
this diode having a 12.5 W power dissipation.
2. The Common Mode Choke Coil and the capacitors C1 and C2 are used to
reduce the conducted noise coming to the RA-RFM via the supply lines.
3. All input and output signals should be protected with 5.6 V Zener diodes. The
specified type can dump 1.3 W.
4. The coils L1 to L6 are ferrite beads and should put in series to the line when
conducted noise is observed entering via the interface lines.
5. The varistor V1 protects the antenna circuit against high voltage induced at the
antenna coil, for example by lightning. The type of varistor given is commonly
available but may not be sufficient for protection in all cases.
Note:
The Zener diodes types given in Figure 13 are commonly used types,
not special suppresser diodes for fast voltage spike suppression. If the
application requires it, special suppresser diodes should be used.
Appendix 6